1. Field of the Invention
The present invention generally relates to communication system condition monitoring and maintenance and, more particularly, to location of shielding faults in cable communication systems by detection of signal ingress into shielded cables of a cable communication system.
2. Description of the Prior Art
Cable communication systems for distribution of television programming and other data services are now familiar and popular in the United States. In recent years, so-called upstream signaling arrangements have also been provided on such systems for providing network (e.g. Internet) access for computers, interactive games and the like. Even more recently, it has been proposed to also provide telephone service over the same communication links as well and many cable service providers are implementing and actively marketing such an additional service.
While such cable communications systems were initially implemented largely with coaxial cable comprising a principal, centrally located signal conductor and a metal shielding layer at or near the periphery of the cable, such distribution systems now commonly use fiber-optics for the principal portion of the distribution network but retain the use of coaxial cable for final distribution to relatively small groups of customers (about 200-1200 customers per group). One of many reasons for such a substitution is that fiber-optic links are not generally subject to signal egress or pick-up of environmental noise while coaxial cable suffers from such effects at any flaw such as an opening or irregularity in the shielding layer. Since the portion of the frequency spectrum used for cable communications overlaps with the frequency band used for aircraft navigation, signal egress from cable communication systems has been a major concern for cable system operators and detection of signal egress has been the mechanism of choice for detecting and locating shielding flaws.
However, signal ingress through such shielding flaws is becoming much more of interest than signal egress for several reasons. Signal egress is currently becoming much less of a problem in regard to interfering with other communications due to the reduction of the extent of use of coaxial cables in cable distribution networks whereas upstream signaling involves use of amplifiers and modulated lasers to couple signals originating the coaxial cable portions of the network into fiber-optic portions of the network and ingress of signals at excessive levels can overload such lasers by driving them into non-linear regions of operation (e.g. saturation) which greatly increases noise while often blocking or at least effectively attenuating the transmission of information-bearing signals (e.g. greatly decreasing the signal to noise ratio (SNR) beyond where information can be reliably communicated) and possibly causing damage to the lasers.
Additionally, increasing use of digital transmission has made egress signals much more difficult to detect since the signals tend to more closely resemble noise and only the carrier signal frequency can be used for reliable detection of signal egress. Moreover, the proposed standards for digital television signal transmission propose a signal format in which the carrier frequency is suppressed, while signal levels are greatly reduced and spread over a frequency band of 2-6 MHZ, depending on the transmitted bit rate; leaving virtually no portion of the signal by which an egress signal can be identified as such even though likelihood of interference from egress signals that has been a concern in the past is reduced. Therefore, ingress of signals into a cable communication system is becoming of increased concern while the importance of signal egress is diminishing.
Nevertheless, signal egress and ingress are symmetrical effects due to the same types of flaws in shielding of the coaxial cables employed in cable communication systems which must be monitored and repaired to maintain communication system efficiency and proper performance. While detection of signal egress has been effective in the past for detection of such flaws, changes in the signals communicated have made it considerably less so and circumstances which may make flaw detection by signal egress detection impractical if not impossible are certainly foreseeable. Conversely, while signal ingress is becoming of much greater importance, the ability to detect shielding flaws through which signal ingress can occur are diminishing while the intentional production of signal ingress has been impractical for a number of reasons (particularly for system maintenance purposes since it does not, itself, provide closed-loop feedback for location of flaws which is available when detecting signal egress) and certainly contraindicated in view of the malfunctions that may be caused in cable communication systems of current designs capable of providing upstream signaling.